THE VERY DEEP ORIGIN OF THE WORLD'S BIGGEST DIAMONDS

Large and relatively pure diamonds like the historic 3,106 ct Cullinan, found in South Africa in 1905, have long been regarded as unusual based on their physical characteristics. For example, they often exhibit exceptional color and clarity, while routinely qualifying as type IIa, a rare designation of chemical purity. A new research discovery about these Cullinan-like diamonds is that they contain heretofore unknown, deeply derived inclusions that originate below the continental mantle keel and are thus known as superdeep diamonds (Smith et al., 2016). Originating from a depth between 360 and 750 km, they reveal information about the conditions within the convecting mantle, beneath the earth's rigid tectonic plates. Here we review the previously published findings, compare the Cullinan-like diamonds to the more abundant lithospheric diamond population, and offer evidence from some additional diamond samples that further verifies their superdeep origin. Cullinan-like diamonds contain minute and rare silicate and iron-rich metallic inclusions surrounded by a fluid jacket composed of methane and hydrogen. The inclusion compositions suggest that this deep mantle environment contains small pockets of oxygen-deficient metallic liquid out of which the diamonds crystallized. This new and unexpected observation made on the world's most expensive diamonds is important for understanding the chemical reactions between mineral assemblages in the deep earth. It shows that deep regions of the mantle contain metallic iron, as opposed to the shallower, more oxidized mantle rocks actively participating in plate tectonics and its associated volcanism.